As an example a screw pump including first, second and third screw type rotors, JP1986-294178A (hereinafter, referred to as reference 1) discloses a screw pump including a main rotor, which is accommodated in a main accommodating bore formed at a casing, and two sub rotors, which are respectively accommodated in two sub accommodating bores formed at the casing so as to be in parallel with the main accommodating bore. A helical thread (i.e., a helical tooth portion and a helical groove portion) is formed at each of the main rotor and the two sub rotors. Each sub rotor meshes the main rotor, so that each sub rotor is driven to rotate.
Generally, a distance between a central axis of the main accommodating bore and a central axis of each sub accommodating bore, a root circle diameter (thread root circle diameter) of the main rotor, and a pitch circle diameter (thread top circle diameter) of each of the sub rotors are arranged to match one another. In such condition, ideally, a cross sectional shape of a flank surface of the main rotor, which is taken in a direction perpendicular to an axial direction of the main rotor, is formed along an epicycloid traced by an edge of the sub rotor (i.e., by a boundary point between a flank surface and a thread top surface of the sub rotor). The epicycloid may be defined as a specific kind of epitrochoid. However, in the description enclosed herein, the term “epitrochoid” does not include the meaning of “epicycloid”.
Further, according to a known screw pump including first, second and third screw type rotors, each of the above described three dimensions (i.e., a distance between the central axes of a main accommodating bore and a sub accommodating bore, a thread root circle diameter of a main rotor and a thread top circle diameter of each sub rotor) is generally determined to be 0.6 times longer than a pitch circle diameter (thread top circle diameter) of the main rotor, while a root circle diameter (thread root circle diameter) of each sub rotor is determined to be substantially 0.2 times longer than the thread top circle diameter of the main rotor.
Still further, as another example of a known screw pump including a main rotor and two sub rotors, U.S. Pat. No. 7,234,925B2 (hereinafter, referred to as reference 2) discloses a screw pump, in which the above described dimensions (i.e., a distance between central axes of a main accommodating bore and a sub accommodating bore, a thread root circle diameter of a main rotor and a thread top circle diameter of each sub rotor) are not arranged to match one another. Specifically, the reference 2 discloses that the thread root circle diameter of each sub rotor is preferably determined to be less than 0.31 times the length of the thread top circle diameter of the main rotor. In such condition, a cross sectional shape of a flank surface of the main rotor, which is taken in a direction perpendicular to an axial direction of the main rotor, is formed along an epitrochoid. Further, the thread root circle diameter of the main rotor is determined to be larger than a distance between an axis of the main rotor and an axis of the sub rotor.
However, according to the reference 1, when seen in a cross section, because the epicycloids is employed as a theoretical line along which the flank surface of the main rotor is to be formed, a corner portion is formed at a connecting portion between the flank surface and a thread root portion of the main rotor. Accordingly, forming the thread portion along the theoretical line may be difficult. Specifically, at least a minimum corner portion R is required to be formed at the connecting portion between the flank surface and the thread root portion of the main rotor, and such corner portion R is accordingly required to be formed at the edge of the sub rotor. So configured, the shape of the flank surface of the sub rotor may become further different from its theoretical shape. Accordingly, a clearance generated between the main rotor and each sub rotor become larger when the main rotor and each sub rotor mesh with each other, so that leakage of the fluid may be increased.
Further, as a processing manner, a roll-forming process is known to be more advantageous than a cutting (grinding) process in processing accuracy and processing time. However, according to the reference 1, the flank surface of the sub rotor is recessed, in a rotational direction thereof, further than a line connecting a center and the edge of the sub rotor. Thus, the flank surface of the sub rotor is formed to have an undercut shape. Accordingly, the roll-forming process does not suit the forming of the sub rotor.
Still further, because the thread root circle diameter of the sub rotor becomes smaller, a resistibility of the sub rotor may be reduced specifically when downsizing the screw pump. Accordingly, in case where the sub rotor is formed by the roll-forming process, the material of the sub rotor may be deformed or damaged. Due to such circumstances, the processing manner of the rotors may be therefore restrained.
In a condition where the thread root circle diameter of the main rotor is determined to be larger than the distance between the axes of the main accommodating bore and the sub accommodating bore as disclosed in the reference 2, the flank surface of the main rotor is formed along the epitrochoid, and the flank surface and the thread root of the main rotor are continuously (smoothly) connected to each other. Accordingly, the shape of the flank surface of the sub rotor approaches the theoretical line, so that the leakage of fluid may be restrained from increasing. However, the reference 2 does not disclose a rotor, of which sub rotor includes a flank surface without an undercut shape and which is suitable to be formed by a roll-forming process.
A need thus exists for a screw pump, which is not susceptible to the drawback mentioned above.